Polyethylene terephthalate and its copolyesters (hereinafter referred to collectively as “PET”) arc widely used to make containers for carbonated soft drinks, juice, water, and the like due to their excellent combination of clarity, mechanical, and gas barrier properties. In spite of these desirable characteristics, oxygen and carbon dioxide gas barrier properties of PET limit application of PET for smaller sized packages, as well as for packaging oxygen sensitive products, such as beer, juice, and tea products. A widely expressed need exists in the packaging industry to further improve the gas barrier properties of PET.
The relatively high permeability of PET to carbon dioxide limits the use of smaller PET containers for packaging carbonated soft drinks. The permeation rate of carbon dioxide through PET containers is in the range of 3 to 14 cc's per day or 1.5 to 2 percent per week loss rate at room temperature depending on the size of the container. A smaller container has a larger surface area to volume ratio resulting in a higher relative loss rate. For this reason, PET containers are currently used only as larger containers for packaging carbonated soft drinks, while metal cans and glass containers are the choice for smaller carbonated soft drink containers.
The amount of carbon dioxide remaining in a packaged carbonated soft drink determines its shelf life. Normally, carbonated soft drink containers are filled with approximately four volumes of carbon dioxide per volume of water. It is generally accepted that a packaged carbonated soft drink reaches the end of its shelf life when 17.5 percent of the carbon dioxide in the container is lost due to permeation of the carbon dioxide through the container side wall and closure. After a PET bottle is filled with approximately four volumes of carbon dioxide, the PET bottle will slowly expand with time due to the creep of PET molecules under pressure. The carbonation level is reduced due to the bottle expansion. The permeability of PET to carbon dioxide and the degree of bottle expansion due to PET molecule creep therefore determines the shelf life of the packaged carbonated beverage and thus, the suitability of PET as a packaging material.
Numerous technologies have been developed or are being developed to enhance the barrier of PET to small gas molecules, but some are too expensive and others may cause undesirable changes in PET mechanical properties, stretch ratio, and/or clarity. Very little work has been done to improve PET bottle carbonation shelf life by controlling PET bottle creep.
Thus, there is a need in the art to enhance the barrier performance of PET and control PET bottle creep for use in applications that will require enhanced barrier, such as in the packaging of carbonated beverages and oxygen sensitive beverages and foods, in a manner that does not cause substantial degradation of the PET mechanical properties, does not substantially impact the stretch ratio of the PET, and/or does not negatively impact the clarity of the PET.